Space Experimental Investigation On Thermocapillary Migration Of Bubbles

Results from a space experiment on bubble thermocapillary migration conducted on board the Chinese 22nd recoverable satellite were presented. Considering the temperature field in the cell was disturbed by the accumulated bubbles, the temperature gradient was corrected firstly with the help of the temperature measurement data at six points and numerical simulation. Marangoni number (Ma) of single bubble migrating in the space experiment ranged from 98.04 to 9288, exceeding that in the previous experiment data. The experiment data including the track and the velocity of two bubble thermocapillary migration showed that a smaller bubble would move slower as it was passed by a larger one, and the smaller one would even rest in a short time when the size ratio was large enough.

Wind Tunnel Experimental Investigation Of Sand Velocity In Aeolian Sand Transport

Sand velocity in aeolian sand transport was measured using the laser Doppler technique of PDPA (Phase Doppler Particle Analyzer) in a wind tunnel. The sand velocity profile, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence were analyzed in detail. The experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0.01 in, while a deviation occurs below 0.01 m. The mean vertical velocity of grains generally ranges from -0.2 m/s to 0.2 m/s, and is downward at the lower height, upward at the higher height. The probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right-skewed at a height of 4 turn in the lower part of saltation layer. The vertical profile of the horizontal RMS velocity fluctuation of particles shows a single peak. The horizontal RMS velocity fluctuation of sand particles is generally larger than the vertical RMS velocity fluctuation. The RMS velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity. The particle turbulence intensity decreases with height. The present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity. (C) 2007 Elsevier B.V All rights reserved.

Investigation Of Mode Ii Crack Growth Following A Very High Speed Impact

A recoverable plate impact testing technology has been developed for studying fracture mechanisms of mode II crack. With this technology, a single duration stress pulse with submicrosecond duration and high loading rates, up to 10(8) MPam(1/2)s(-1), can be produced. Dynamic failure tests of Hard-C 60# steel were carried out under asymmetrical impacting conditions with short stress-pulse loading. Experimental results show that the nucleation and growth of several microcracks ahead of the crack tip, and the interactions between them, induce unsteady crack growth. Failure mode transitions during crack growth, both from mode I crack to mode II and from brittle to ductile fracture, were observed. Based on experimental observations, a discontinuous crack growth model was established. Analysis of the crack growth mechanisms using our model shows that the shear crack extension is unsteady when the extending speed is between the Rayleigh wave speed c(R) and the shear wave speed c(S). However, when the crack advancing speed is beyond c(S), the crack grows at a steady intersonic speed approaching root 2c(S). It also shows that the transient mechanisms, such as nucleation, growth, interaction and coalescence among microcracks, make the main crack speed jump from subsonic to intersonic and the steady growth of all the subcracks causes the main crack to grow at a stable intersonic speed.

Experimental Investigation Of Influence Of Interfacial Tension On Convection Of Two-Layer Immiscible Liquid

Benard-Marangoni convections of two-layer fluids heated from the bottom are investigated experimentally with a particle imagine velocimetry. The flows are visualized from the side, and various velocity fields near the onset of convection, such as three-layer vortex convective patterns, are observed when the depth ratio varies in a wide range. A new classification of the convective patterns is proposed with more detail than in previous studies. The analysis of the results indicates that the interface tension greatly influences the motion intensities of the bottom and top layers. The dimensionless wave number increases with the Bond number when the motion in the top layer is not more intense than that in the bottom layer, which agrees with the theoretical prediction.

Experimental investigation on the cavity-based scramjet model

The present work focused on improving the engine performance with different fuel equivalence ratios and fuel injections. A scramjet model with strut/cavity integrated configurations was tested under Mach 5.8 flows. The results showed that the strut may sreve as an effective tool in a kerosene-fueled scramjet. The integration of strut/cavities also had great effect on stablizing the combustion in a wide range of fuel equivalence ratio. The one-sdimensional analysis method was used to analyze the main characteristics of the model. The two-stage fuel injection should have better performance in increasing the chemical reaction rate in the first cavity region.

An investigation of electromagnetic wave propagation in plasma by shock tube

This paper presents the electromagnetic wave propagation characteristics in plasma and the attenuation coefficients of the microwave in terms of the parameters n(e), v, w, L, w(b). The phi800 mm high temperature shock tube has been used to produce a uniform plasma. In order to get the attenuation of the electromagnetic wave through the plasma behind a shock wave, the microwave transmission has been used to measure the relative change of the wave power. The working frequency is f = (2 similar to 35) GHz (w = 2pif, wave length lambda = 15 cm similar to 8 mm). The electron density in the plasma is n(e) = (3 x 10(10) similar to 1 x 10(14)) cm(-3). The collision frequency v = (1 x 10(8) similar to 6 x 10(10)) Hz. The thickness of the plasma layer L = (2 similar to 80) cm. The electron circular frequency w(b) = eB(0)/m(e), magnetic flux density B-0 = (0 similar to 0.84) T. The experimental results show that when the plasma layer is thick (such as L/lambda greater than or equal to 10), the correlation between the attenuation coefficients of the electromagnetic waves and the parameters n(e), v, w, L determined from the measurements are in good agreement with the theoretical predictions of electromagnetic wave propagations in the uniform infinite plasma. When the plasma layer is thin (such as when both L and lambda are of the same order), the theoretical results are only in a qualitative agreement with the experimental observations in the present parameter range, but the formula of the electromagnetic wave propagation theory in an uniform infinite plasma can not be used for quantitative computations of the correlation between the attenuation coefficients and the parameters n(e), v, w, L. In fact, if w < w(p), v(2) much less than w(2), the power attenuations K of the electromagnetic waves obtained from the measurements in the thin-layer plasma are much smaller than those of the theoretical predictions. On the other hand, if w > w(p), v(2) much less than w(2) (just v approximate to f), the measurements are much larger than the theoretical results. Also, we have measured the electromagnetic wave power attenuation value under the magnetic field and without a magnetic field. The result indicates that the value measured under the magnetic field shows a distinct improvement.

Investigation on mechanical properties and size effect of nanocrystalline twin copper

The stress-strain relations of nanocrystalline twin copper with variously sized grains and twins are studied by using FEM simulations based on the conventional theory of mechanism-based strain gradient plasticity (CMSG). A model of twin lamellae strengthening zone is proposed and a cohesive interface model is used to simulate grain-boundary sliding and separation. Effects of material parameters on stress-strain curves of polycrystalline twin copper are studied in detail. Furthermore, the effects of both twin lamellar spacing and twin lamellar distribution on the stress-strain relations are investigated under tension loading. The numerical simulations show that both the strain gradient effect and the material hardening increase with decreasing the grain size and twin lamellar spacing. The distribution of twin lamellae has a significant influence on the overall mechanical properties, and the effect is reduced as both the grain size and twin lamellar spacing decrease. Finally, the FEM prediction results are compared with the experimental data.

Numerical investigation of thermocapillary migration of the drop for large Marangoni numbers

An axisymmetric model is adopted to simulate the problem of unsteady drop thermocapillary motion for large Marangoni numbers. Front tracking methods are used in the investigation. It is found that the non-dimensional drop migration velocity will decrease with increasing Marangoni number. This agrees well with the experimental results obtained from the 4th Shen-Zhou space ship. In the meanwhile, this is also the first time for numerical simulations to verify the experimental phenomenon under large Marangoni numbers.

Experimental investigation on bubble coalescence under nonuniform temperature distribution in reduced gravity

Results on bubble coalescences from the space experiment of thermocapillary bubble migration conducted on board the Chinese 22nd recoverable satellite are presented in this paper. Some coalescences of large spherical bubbles under microgravity are observed through bubbles staying at the upper side of the test cell. The data of bubble coalescence time are recorded and compared with theoretical predictions, which is based on a theory to describe the tendency of coalescence connected to chemical potential difference. It is implied that the theory is applicable for the experimental data of bubble coalescence. Moreover, the angle between the line of two bubble centers and temperature gradient falled mostly in the range 20 degrees-40 degrees. (C) 2007 Elsevier Inc. All rights reserved.

Atomistic investigation of the effects of temperature and surface roughness on diffusion bonding between Cu and Al

Molecular dynamics (MD) simulations are carried out to analyze the diffusion bonding at Cu/Al interfaces. The results indicate that the thickness of the interfacial layer is temperature-dependent, with higher temperatures yielding larger thicknesses. At temperatures below 750 K, the interface thickness is found to increase in a stepwise manner as a function of time. At temperatures above 750 K, the thickness increases rapidly and smoothly. When surface roughness is present, the bonding process consists of three stages. In the first stage, surfaces deform under stress, resulting in increased contact areas. The second stage involves significant plastic deformation at the interface as temperature increases, resulting in the disappearance of interstices and full contact of the surface pair. The last stage entails the diffusion of atoms under constant temperature. The bonded specimens show tensile strengths reaching 88% of the ideal Cu/Al contact strength. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.